Subsea well production risers typically are made up of sections of pipe joined together with threaded connectors. In one type of production riser, the threaded connectors are formed separately and welded to the ends of pipe. These risers are subject to large bending forces due to wave motion, wind and currents. As a result, the risers are subject to severe fatigue and tensile loadings from the bending and weight of the riser string. Generally, the weakest point is at the weld.
One of the problems associated with welding the threaded connectors to the pipe is that often the pipe will not be precisely dimensioned at the weld. The pipe may be out of round or have an inner diameter that is too small or too large. The connector is a machined piece, thus its weld end is very accurate with respect to roundness and inner diameter. A typical tolerance might be plus or minus 0.010″, although it could be smaller. It is very expensive to specify such high tolerances for pipe from a steel mill. For conventional API pipe, the inner diameter is governed by tolerances on the outer diameter and wall thickness, and is held to approximately plus or minus 2%. In special cases, mills have held inner diameter tolerances to about plus or minus 0.062″ at a significant increase in cost.
Consequently, a mismatch at the inner diameters of the pipe and connector is likely to occur and result in inferior welds. This mismatch may lead to an early failure of the weld due to stress concentrations at the point of mismatch. Any mismatch on the exterior can be ground smooth, but a grinding operation in the inner diameter after welding would be expensive and time consuming.
In the prior art of machining threads directly on pipe ends, it is known to perform sizing operations on pipe ends prior to machining the threads. To applicant's knowledge, however, no one has performed sizing operations on a pipe end to achieve a tight tolerance inner diameter, then butt welding the pipe end to a threaded connector.